It is known that optical fibers in which the core is doped with particular substances, for example, rare-earth ions, have stimulated emission characteristics adapted to be used as laser sources and in optical amplifiers.
In fact, these fibers can be supplied with a light energy at a particular wavelength, which is capable of bringing the doping substance atoms to an excited energy state, or pumping band, from which the atoms spontaneously decay in a very short time to a laser emission state in which state they remain for a relatively longer time.
When a fiber having a high number of atoms excited in the emission level is traversed by a light signal with a wavelength corresponding to such laser emission state, the light signal causes the transition of the excited atoms to a lower level and the light emission has the same wavelength as the signal. Therefore, a fiber of this kind can be used to obtain optical signal amplification.
Starting from the excited state, the atom decay can occur also spontaneously which gives rise to a random emission constituting a "background noise" overlapping the stimulated emission, corresponding to the amplified signal.
Such phenomena takes place at several wavelengths, typical of the doping substance, so as to give origin to a fluorescence spectrum. In order to obtain the maximum signal amplification by means of a fiber of the above type, together with a high signal-to-noise ratio, normally, the art uses a signal with a wavelength corresponding to a maximum of the fluorescence spectrum curve of the fiber incorporating the doping substance used. This signal usually is generated by a laser emitter or light source.
For example, such a fiber can be used as an amplifying fiber when the core of the fiber is doped with aluminum (Al.sup.3+) and erbium ions (Er.sup.3+), as described in U.S. application Ser. No. 07/363,072, filed Jun. 8, 1989. However, the erbium fluorescence spectrum, in the range Of the concerned wavelengths, has a particularly narrow emission peak which dictates the use, as a transmission signal source, of a laser emitter operating at a well defined wavelength with a limited tolerance because signals exceeding such tolerance would not be properly amplified and at the same time a strong amplification of the background noise would occur at this wavelength.
However, laser emitters or light sources having the required precision in wavelength are difficult and expensive to produce, whereas with the common industrial production of these devices, there is a rather large tolerance with respect to the emission wavelength.
While in some applications, such as, for example, submarine telecommunication lines, transmission signal emitters operating at the right wavelength could be used, for example, obtained through selection from commercially available lasers, so as to use only those having an emission strictly close to the laser emission peak of the amplifier fiber, this procedure is not acceptable from an economical point of view when other kinds of lines are concerned, such as, for example, municipal communication lines when it is of great importance to limit the installation costs.
For example, a fiber according to said application Ser. No. 07/363,072 having been doped with aluminum ions for modifying the index of refraction and with erbium ions to provide the laser emission has an emission peak of about 1,531 nm, and for a range of .+-.5 nm from this value provides a high intensity of emission and could be used for amplification. Therefore, for operating with this optical fiber, it is better to use a communication signal in the same wavelength range. However, commercially available semiconductor lasers, which could be suitable for the use, are usually made with emission wavelength values in the range of 1,520 to 1,570 nm.
As a result, a great number of commercially available lasers have wavelengths outside of the desired range and, therefore, cannot generate a signal adapted to be properly amplified.
On the other hand, it is known that erbium-doped fibers have an area in the emission spectrum with a high and substantially constant intensity in the wavelength range contiguous to the above described peak and comprising the signal range of the above mentioned commercially available lasers. However, in this optical fiber, a signal supplied at a wavelength removed from the maximum of the emission peak would be amplified in a reduced measure, whereas spontaneous transitions from the laser emission state in the fiber mainly take place with the emission at the wavelength of the spectrum peak, at 1,531 nm, thereby generating a "background noise" which will be further amplified through the fiber length and will overlap the useful signal.
It may be envisioned that filtering of the light emission constituting "the noise" at the end of the amplifier could be used, accepting therefore only the wavelength of the signal, which would require providing a suitable filter at the end of the active fiber. However, the presence of an undesired emission in the fiber in the range of the fiber maximum amplification would absorb pumping energy thereby making the fiber substantially inactive with respect to the communication signal amplification itself.
Interference filters are also known and can be disposed at different locations along the amplification fiber, but known filters of this kind are formed by discrete components, not made of fiber, and therefore, they need light beams in the air which make them unsuitable for industrial application.
The problem arises therefore of providing an active optical fiber to be employed in optical amplifiers which is adapted to be used together with commercially available lasers for the emission of the transmission signal without further restrictions.